1. Field of the Invention
The present invention relates to an inductive load driving device.
2. Description of Related Art
There has been proposed various methods, which lower noise generated when a plurality of inductive loads such as motors is simultaneously driven by pulse width modulation (PWM) signals. For example, in cases where two motors are driven by two MOSFET, there has been proposed a method, which lowers switching noise by avoiding the simultaneous switching of the two MOSFET (reference to JP-A-H10-2222). With this method, the PWM driving signal input to one MOSFET is out of phase with the PWM driving signal input to the other MOSFET, and the phases are set such that the end timing of rising edge of one pulse conforms to the start timing of trailing edge of the other pulse, in order to avoid the simultaneous switching of the two MOSFET.
With such an inductive load driving device, the switching noise of MOSFET is lowered, but harmonic noise generated in the changing points of the two PWM driving signals can not be sufficiently lowered. More particularly, when the two PWM driving signals are adopted to be a PWM driving signal A and a PWM driving signal B, and looking at the temporary changes in one cycle of the both signals, the seven changing points exist, such as the start point of rising edge of the PWM driving signal A→the end point of rising edge of the PWM driving signal A (=the start point of trailing edge of the PWM driving signal B)→the end point of trailing edge of the PWM driving signal B→the start point of trailing edge of the PWM driving signal A→the end point of trailing edge of the PWM signal A→the start point of rising edge of the PWM driving signal B→the end point of rising edge of the PWM driving signal A, and harmonic noise is generated in each of the changing points as shown in FIG. 14. Therefore, the noise that the frequency component is further increased is generated in its entirety.